This proposal continues ongoing analysis of the mechanisms by which neural circuits in primary visual cortex represent stimulus features. Work from this lab has been exploring the contribution of feedforward projections from layer 4 and recurrent connections within layer 2/3 to the orientation selective responses of layer 2/3 neurons. The experiments proposed here will expand on this line of inquiry by investigating how other properties of the stimulus such as luminance and color, and how multiple stimulus orientations, are represented in the responses of orientation selective layer 2/3 neurons. As demonstrated by preliminary results, changes in background luminance exert a powerful suppressive influence over the response of orientation tuned layer 2/3 neurons, dramatically delaying the emergence of a tuned response and virtually eliminating an ongoing tuned response. The first aim explores the feedforward and recurrent mechanisms that contribute to this effect. The second aim probes whether the mechanism that is responsible for representing edge orientations at short wavelengths of light is distinct from that responsible for representing edge orientations at longer wavelengths. Specifically, experiments will test whether the short wavelength-sensitive cone system, which preliminary results show is capable of driving orientation selective responses in layer 2/3, does so independent of feedforward inputs from layer 4. Finally, the third aim examines how two edge orientations presented simultaneously are represented in the responses of layer 2/3 neurons. Preliminary results demonstrate that the addition of a second grating stimulus alters the response of layer 2/3 neurons in a highly predictable fashion; the mechanisms that underlie these effects will be explored. The proposed experiments employ intracellular, extracellular, and optical recording techniques to probe these issues at both the single cell and population level. Taken together they should provide new insights into the functional organization of local circuits in visual cortex, information that is crucial for understanding the neural basis of visual perception.